1. Field of the Invention
The present invention relates to electroacoustic transducers for converting electrical signals into sound, such as loudspeakers, and particularly to electroacoustic transducers having a structure effective in reducing the thickness.
2. Description of Related Art
A loudspeaker includes a diaphragm vibrated by supplying a driving current to a coil attached to the diaphragm and applying to the coil a magnetic flux emitted from a direct current magnetic field generator including a magnet.
For example, a conventional loudspeaker of outer magnet type shown in FIG. 31 includes a coil 9 wound into a cylinder, a ring-shaped magnet 90 and a columnar pole 95 located outside and inside the coil 9, respectively, an upper plate 97 attached to the front face of the magnet 90, and a bottom plate 96 attached to the rear face of the pole 95 and magnet 90. In this loudspeaker, the coil 9 is located in a magnetic field formed in a cylindrical gap between the pole 95 and the upper plate 97, so that the coil 9 will be driven.
A conventional loudspeaker of inner magnet type shown in FIG. 32 includes a coil 9 wound into a cylinder, a disk-like magnet 92 and a cup-like yoke 99 located inside and outside the coil 9, respectively, and a plate 98 attached to the front face of the magnet 92. In this loudspeaker, the coil 9 is located in a magnetic field formed in a cylindrical gap between the plate 98 and the yoke 99, so that the coil 9 will be driven.
Another conventional loudspeaker of outer magnet type shown in FIG. 33 includes a coil 91 wound into an angular cylinder, a pair of magnets 93, 93 and a pole 95 each in the form of a rectangular parallelepiped located outside and inside the coil 91, respectively, upper plates 97, 97 attached to the front faces of the magnets 93, 93, and a bottom plate 96 attached to the rear face of the pole 95 and magnets 93. In this loudspeaker, the coil 91 is located in a magnetic field formed in a gap between the pole 95 and the upper plates 97, 97, so that the coil 91 will be driven.
Another conventional loudspeaker of inner magnet type shown in FIG. 34 includes a coil 91 wound into an angular cylinder, a tabular magnet 94 and a box-like yoke 99 located inside and outside the coil 91, respectively, and a plate 98 attached to the front face of the magnet 94. In this loudspeaker, the coil 91 is located in a magnetic field formed in a gap between the plate 98 and the yoke 99, so that the coil 91 will be driven.
However, all of the above conventional loudspeakers have a problem in that they are difficult to make thinner because the coil greatly protrudes beyond the front face of the yoke. Accordingly, there has been proposed a thin loudspeaker shown in FIG. 35 (see JP 3213521, B). This loudspeaker includes a frame 100 having a sound emitting hole 101 and containing a diaphragm 102 having its periphery fixed to the frame 100, a coil 104 having an axis S perpendicular to the diaphragm 102 and attached centrally to the diaphragm 102, and a disk-like magnet 103 located coaxially with the axis S of the coil 104 and magnetized in the direction parallel to the axis. A gap G is formed axially of the coil 104 between the magnet 103 and the coil 104.
In this loudspeaker, a magnetic flux occurs from a surface of the magnet 103 that faces the diaphragm 102, as indicated by broken lines in FIG. 35. The magnetic flux acts on the coil 104 through the gap G. Supplying a driving current to the coil 104 in this state drives the diaphragm 102, which then vibrates axially of the coil 104.
There have been proposed other thin loudspeakers having a similar structure (JP 3208310, B, JP 2005-223720, A). In such thin loudspeakers, the coil has a flat shape where it is wound more in the direction perpendicular to the axis than in the axial direction. This allows making the loudspeakers thinner than those shown in FIG. 31 to FIG. 34.
However, the thin loudspeaker as shown in FIG. 35 still has a problem in that there is an increasing effect on sound pressure drop as it is made smaller/thinner. This is because only a magnetic flux component of the magnetic flux generated from the magnet that is perpendicular to the axis of the coil acts as a driving force for the coil, and a magnetic flux component that is parallel to the axis of the coil does not contribute as a coil driving force.